Electronically commutated motors are of interest in applications requiring high efficiency and reliability, such as compressors for refrigeration, fans and pumps. Such motors consist of a coiled stator and a rotor with permanent magnets, the motor being supplied voltage by an electronic control that energizes the stator coils, according to a sequence and at times defined by a rotor position sensor, which is installed on the shaft, or making use of the voltages induced to the coils by the moving rotor, thereby imparting torque to the motor.
In some applications, such as for example hermetic compressors, the operation of these motors should offer the best energy efficiency, corresponding to the capacity of varying the motor working speed. There are a number of specific driving techniques for obtaining the speed control. One of the known techniques uses voltage variation supplied by the direct current bus of an inverter circuit. This requires a direct current supply unit with an adjustable voltage value, which is a disadvantage, since it is an additional circuit that causes losses to the system and requires active electronic components, which are rather sophisticated and of high cost.
Another solution used for controlling the speed of electronically commutated motors is by modulating the voltage induced to the motor coils, through techniques of the PWM type (pulse width modulation), controlling the average value of the output voltage and consequently the motor speed. This technique, even allowing a continuous speed adjustment through a source of fixed DC voltage, without using additional components and circuits in the system, has the drawback of generating high energy losses. The lower the running speed of the motor in relation to the maximum operative speed in which the motor can work, the higher will be said the energy losses. Such techniques, when applied to the refrigerating hermetic compressors with variable capacity, present during the operation thereof in low speeds, has the disadvantage of high energy consumption. Such energy consumption overcomes the advantage of reducing the energy consumption of said systems obtained by varying the refrigerating capacity of the compressor in the above cited conditions.
The energy losses in a system, according to the prior art including the motor and the electronic driving circuit basically consist of:
losses in the magnetic circuit of the motor, due to the variable magnetic flux, one part being due to the component in the fundamental frequency of the magnetic field, which, on its turn, is proportional to the rotational speed of the shaft, while the other part is due to the high frequency components in the magnetic field generated by the voltage modulation at a high frequency of the motor phases;
Joule losses in the wire of the motor winding, in the ohmic resistance of the electronic switches, inverter and other components provided in series with the current path, these losses being proportional to the square of the current and practically invariable with the shaft speed for a fixed value of the torque applied by the motor;
Joule losses in the inverter switches, due to switching and proportional to the frequency at which switching occurs; and
Joule losses in the remaining components of the command circuit, which are fixed for any speed or torque supplied by the motor.
An important amount of the energy losses of the system, according to the prior art, is that resulting from voltage modulation on the motor phases for speed control. FIG. 2 illustrates the shape of the voltage wave A, modulated in pulse width. In this modulation technique, by using a fixed voltage whose value is A, there are obtained average voltages B, which may vary from zero to A, depending on the relation between the time periods t1 and t2, representing the average time for the voltage A to be applied within the total period.
B=A. t1/t2
By modulating the voltage on the motor phases, a speed control is thus obtained, but with the disadvantage of presenting an increase of energetic losses. The lowest the running speed of the motor in relation to the maximum operative speed in which said motor can work, the higher will be said energy losses. Due to the high harmonic content of the voltage induced to the motor phases, causing losses specially when the operation is at a low speed, the efficiency of the system is reduced.